1. Field of the Invention
The present invention relates to a cooling construction for a battery module system that is a combination of at least two non-aqueous electrolyte secondary cells employing parallelepiped cell cases.
2. Description of Related Art
Since lithium ion cells, that may be considered representative of non-aqueous electrolyte secondary cells, have high specific energy density, they are under consideration as cells for batteries for electric automobiles and as cells for energy storage. In particular, as types of electric automobiles, there are a zero emission electric automobile that is equipped with a battery but not with any engine, a hybrid electric automobile that is equipped with both an engine and a secondary battery, and also a plug-in type electric automobile in which the battery is directly charged with electricity derived from an electrical grid. Furthermore, in the field of energy storage, there is a stationary type energy storage system that supplies electrical power in exceptional circumstances when the normal electrical grid power system experiences a power cut.
For these various applications, excellent durability is demanded for a lithium ion cell. In other words there is a demand for a lithium ion cell with low reduction rate of charging capacity and with high retention rate of charging capacity over a long time period, even when the temperature of the surroundings becomes high. However, when a lithium ion cell is used in a system other than an electric automobile in which charging and discharging at high current are required, such as a railroad vehicle or a construction machine or the like, the amount of Joule heat generated in the interior of the cell becomes great, and due to this heat it is quite easy for the electrolyte to be decomposed, or for collapse of the electrode construction to occur. In particular, with a battery module that is made up of a plurality of lithium ion cells, it is difficult to ensure sufficient heat dissipation area from the front surface of the battery, and heat can easily build up in the interior of the module.
As a result, when the battery reaches a high temperature, performance reduction can occur such as decrease of output or of capacity. Accordingly, if a lithium ion battery is used in an application for which charge and discharge at high current is required, cooling of the lithium ion battery becomes very important.
For cooling of a lithium ion battery, apart from air cooling, liquid cooling using a liquid whose specific heat is greater than that of air has also been investigated, and the following per se conventional techniques are known.
In Japanese Laid-Open Patent Publication Heisei 7-122293, there is disclosed a method of cooling a battery by providing it with aluminum fins that are formed as equilateral triangles. In Japanese Patent 3,322,321, there is disclosed a method of cooling a battery by providing a hollow portion equipped with fins that communicates with the exterior of the battery, and by flowing air through the interior thereof. In Japanese Laid-Open Patent Publication Heisei 8-096837, there is disclosed a method in which unit cells are housed internally to a battery module, and cooling is performed by flowing cooled electrolyte over the surfaces of these unit cells. And in Japanese Laid-Open Patent Publication 2000-348781 there is disclosed a technique of covering the positive and negative terminals of a cell with a terminal cooling jacket, and of cooling the positive and negative terminals by circulating a cooling medium in this terminal cooling jacket. Moreover, in Japanese Laid-Open Patent Publication 2006-278330 and in Japanese Laid-Open Patent Publication 2009-009889, there is disclosed a technique of inserting a heat dissipation plate, in which a flow conduit is formed, in the gap between two unit cells, and of cooling the battery by flowing a cooling medium in this flow conduit.